LU-8579 osd-ldiskfs: code cleanup for osd_ldiskfs_add_entry

LU-8579 osd-ldiskfs: code cleanup for osd_ldiskfs_add_entry

Commit 07660ad33a7d1 can give more warning message when directory
size exceeds limitation. But some variables that are allocated
on stack are unnecessary and may cause overflow.

Further more, the "osd_thread_info::oti_dev" is obsolete, should
not be used any more. The fid_seq_is_mdt0() is used against zero
FID, will be replaced by checking whether the device is for MDT0.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Idfe2a4546535b3863e84ce5753b8a86c369dadae
Reviewed-on: http://review.whamcloud.com/22307
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Yang Sheng <yang.sheng@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>